In article <570 at copper.UUCP> mikeb at copper.UUCP (Mike Beckerman) writes:
>In article <15525 at ucbvax.BERKELEY.EDU> ballou at brahms.UUCP (Kenneth R. Ballou) writes:
>>In article <86900030 at haddock> karl at haddock writes:
>>>
>>>main() {
>>> char *foo = "outer";
>>> for (;; printf(foo),exit(0)) {
>>> char *foo = "inner";
>>> }
>>>}
>>>
>>>This prints "outer" (vax SVR2 compiler), though the for-while equivalence
>>>might lead one to expect "inner".
>>
>>I don't think the issue here is equivalence of for and while statements.
>>The point is that the scope of the inner 'foo' is the compound statement
^^^^^^^^
>>which is the body of the for statement. So, quite rightly, the 'foo'
>>given as the argument to printf in the third expression of the for statement
>>refers to the most closely nested declaration of 'foo' -- the body of the
>>for statement is one block level higher and is not visible at this point.
^^^^^^^^^^^^^^^^^^
>
>That was my first thought as well, but both K&R and the proposed ANSI C standard
>define the "for" loop as follows:
>
> for (expression-1 ; expression-2 ; expression-3) statement
>
> is equivalent to
>
> expression-1;
> while (expression-2) {
> statement
> expression-3;
> }
>
>which to me says that the example should have printed "inner".
I think the mistake being made here is a confusion with scope. Yes,
that's the definition of while and for, but look carefully at the
definition for 'while': it has { statement } (brackets surrounding
the statement) after the conditional, while the for has no brackets.
Therefore, the correct translation of your for loop into a while loop
is as follows:
main() {
char *foo = "outer";
while (TRUE)
{
{
char *foo = "inner";
}
printf(foo);
exit(0)
}
}
The for loop execution order is /* for (exp1; exp2; exp3) statement; */
exp1; <=> ;
exp2; <=> ;
statement; <=> { char *foo = "inner" }
exp3; <=> printf(foo); exit(0);
The 'char *foo = "inner"' disappears when the deeper scope gets popped,
therefore the foo that gets printed is still "outer".
Shannon Nelson
"Where there is a will, there's lot's of relatives...'